Disable debug preferences

[Trim]

It disable all preferences modification made by the default/preferences/debug.pref.js.

I didn't remove the file directly, because I wanted to keep information about which configs orignal others have found interesting to modify.

As suggested in the Ericsson#506 pull request, I've added a preference choice to enable/disable the Thunderbird global index:

I've looked at the Thunderbird preference to add an information directly in the "Thunderbird Prefrences dialog > Advanced > Advanced Configuration". But I failed to add the information there, because I wasn't able to do all the path through ids to add an element after the global indexer checkbox.

PS: Please note you have to try this fix in a en-US Thunderbird installation, because we need to create labels in all languages before it could work. So:

• test this patch, you'll need to change the locale with an extension, like "Simple Locale Switcher"
• Before releasing, we'll need to push new strings to Transifex and get back them into all languages.

[advancingu]

Thanks @Trim. I haven't looked at the changes yet but you can safely delete unneeded files, even if they were interesting as reference. You can always go back in Git and look at them again there.